Optical inspection is commonly used in semiconductor device manufacturing to detect defects on the surface of a wafer, such as contaminant particles, scratches and digs. Undetected defects can cause device failures, thus reducing substantially the process yield. Therefore, careful inspection is required to verify the cleanliness and quality both of unpatterned wafers and of patterned wafers at various stages in the manufacturing process.
A common method for inspecting semiconductor wafers is to scan a laser beam over the wafer surface, and measure the light scattered from each point on which the beam is incident. One such method, based on dark-field scattering detection, is proposed by Smilansky et al., in U.S. Pat. No. 6,366,690, whose disclosure is incorporated herein by reference. Smilansky et al. describe a wafer inspection system based on an optical detection head that comprises a laser and a number of light sensors, which are fed by fiberoptic light collectors arrayed around the laser. The optical head is positioned over the wafer surface, and the wafer is rotated and translated so that the laser beam scans over the surface. The sensors detect the radiation that is scattered from the surface in different angular directions simultaneously, as determined by the positions of the fiberoptics. The entire wafer surface is thus scanned, one pixel at a time, along a spiral path.
Another dark-field wafer inspection system is described by Marxer et al., in U.S. Pat. No. 6,271,916, whose disclosure is incorporated herein by reference. In this system, a laser beam is directed toward the wafer surface in a normal direction and scans the surface along a spiral path. An ellipsoidal mirror is used to collect the laser radiation that is scattered from the surface at angles away from the normal. Preferably, light scattered within a first range of angles is collected by one detector, while that scattered within a second range of angles is scattered by another detector. The different detector signals are used to distinguish large defects from small defects.